It is known that shining coherent light upon an illumination surface generates a shimmering illumination pattern also known as speckles. A speckle pattern arises from the local interferences generated between the incoming wavefront from a coherent light source and the scattered wavefront from an illumination surface for example as illustrated in FIG. 1. More specifically, the speckle pattern may originate from the superposition of random discrete wavefronts arising from contributing points in the illumination surface.
In the specific case of a multimode waveguide, it is also known that the propagation of coherent light in the core of the waveguide generates a strong speckled distribution of the intensity. The speckle pattern at the waveguide end may be produced by random interference between the various propagation modes.
The resulting random intensity pattern of these illumination systems is a drawback in many applications, e.g. inspection lighting, where the projected speckles are transformed into imaging noise.
A technique for reducing the speckle noise on an illumination surface involves dynamically decorrelating the speckles of the coherent light source (see FIG. 2A), i.e. by time-varying one or all of the following parameters: the polarization, the phase, and the wavelength of the coherent light source.
Another method is to dynamically decorrelate the projected speckles generated by the illumination surface or by the optical projection system (FIG. 2B). Typically, a moving optical element (e.g., a diffuser) is positioned within the optical path between the coherent light source and the illumination surface. The dynamic motion of this optical element in its entirety reduces the spatial coherence of the incident coherent light and thus a reduction of the overall speckle contrast is achieved. In this case, however, significant motion amplitudes are needed and are typically provided by, e.g. the mechanical rotation of a refractive element (U.S. Patent Application Publication No. 2007/0223091, PCT Patent Application Publication No. WO2009133111, and U.S. Pat. No. 6,081,381), the mechanical vibration or displacement of a plane diffuser (U.S. Patent Application Publication No. 2007/0251916, and U.S. Patent Application Publication No. 2011/0267680), screen vibration (U.S. Patent Application Publication No. 2013/0010356 A1), or the use of segmented mirrors (U.S. Pat. No. 7,502,160 B2).
Another method is to dynamically decorrelate the projection speckles by using at least two successive optical elements within the optical path and to move one of these elements in its entirety with respect to the other (FIG. 2C). In this case, much smaller motion amplitudes are needed to achieve the same reduction of the overall speckle contrast on the final illumination surface (“Speckle Removal by a Slowly Moving Diffuser associated with a motionless diffuser, J. Opt. Soc. Am., 61, pp. 847-851, 1971). The two optical elements may be, e.g. a refractive, diffractive or diffusing optical element.
Some of these methods need bulky, distinct optical elements or involve movement of an optical element in its entirety, which may be hard to integrate directly into a laser waveguide illumination system, such as, e.g. (U.S. Pat. No. 7,437,035 and PCT Patent Application Publication No. WO2012146960 A1) where the speckle pattern may be spread over an elongated surface. Others solutions including decorrelation of the coherent light source may not be suitable for certain applications.